Microwave hybrid comprising trough waveguide and balanced mixer utilizing same

ABSTRACT

A microwave hybrid made up of a section of trough waveguide, rectangular waveguide and coaxial transmission lines is provided. The rectangular waveguide and the trough waveguide are joined end to end. The side walls of the trough waveguide form a continuous structure with the broad walls of the rectangular waveguide. The bottom wall of the trough waveguide forms a continuous structure with a narrow wall of the rectangular waveguide. Near the junction region of the trough waveguide and the rectangular section are coupled a pair of coaxial transmission lines with the inner conductor of one of the coaxial transmission lines extending through a first of the side walls of the trough waveguide and the inner conductor of the other coaxial transmission line extending through the second opposite side wall of the trough waveguide.

Unite States atent [is] 3,654,556 Wen 51 Apr. 4%, 1972 [54] MICROWAVE HYBRID COMPRISING Primary Examiner-Herman Karl Saalbach TROUGH WAVEGUIDE AND BALANCED MIXER UTILIZING SAME Assistant Examiner-Paul L. Gensler AttorneyEdward J. Norton [57] ABSTRACT A microwave hybrid made up of a section of trough waveguide, rectangular waveguide and coaxial transmission lines is provided. The rectangular waveguide and the trough waveguide are joined end to end. The side walls of the trough waveguide form a continuous structure with the broad walls of the rectangular waveguide. The bottom wall of the trough waveguide forms a continuous structure with a narrow wall of the rectangular waveguide. Near the junction region of the trough waveguide and the rectangular section are coupled a pair of coaxial transmission lines with the inner conductor of one of the coaxial transmission lines extending through a first of the side walls of the trough waveguide and the inner conductor of the other coaxial transmission line extending through the second opposite side wall of the trough waveguide.

9 Claims, 4 Drawing Figures MICROWAVE HYBRID COMPRISING TROUGH WAVEGUIDE AND BALANCED MIXER UTILIZING SAME This invention relates to microwave hybrid circuits and more particularly to a hybrid configuration employing trough waveguides and rectangular waveguides.

At the present time considerable effort is being expended to develop microwave systems which operate at millimeter wavelengths. Transmission lines fabricated on dielectric substrates such as strip transmission lines and microstrip transmission lines suffer from high dielectric loss and the possibility of mode conversion radiation unless their transverse or cross section dimensions are substantially less than one-half wavelength. Because the transverse dimensions of the lines are small due to operating wavelength dielectric loading (which further shrinks these dimensions), the strip transmission line and the microstrip transmission lines become undesirable at high microwave frequencies. Therefore, at millimeter wavelengths, precision hollow waveguides are more desirable than strip transmission lines from the standpoint of loss.

It is an object of the present invention to provide a single microwave hybrid structure using the combination of symmetrical trough waveguide and rectangular waveguide.

Briefly, this and other objects of the present invention are provided by a symmetrical trough waveguide section joined end to end to a section of rectangular waveguide. The section of symmetrical trough waveguide has a pair of longitudinally extending side walls of conductive material spaced by longitudinally extending conductive bottom wall. A shorter conductive center fin is symmetrically disposed between the pair of side walls and extends from the bottom wall in a manner to form a first trough between the center fin and a side wall and a second trough between the center fin and the opposite side wall. The height of a side wall and the spacing between the side walls is arranged to allow propagation of signals along the symmetrical trough waveguide in even and odd modes. The rectangular waveguide section is joined to the trough waveguide in a manner such that one of the broad walls of the rectangular waveguide is joined to form a first continuous surface with a first of the pair of side walls and such that the other of the broad walls is joined to form a second continuous surface with the second of the pair of side walls. One of the narrow walls of the rectangular waveguide is joined to the bottom wall of the trough waveguide to form a continuous surface with the bottom wall of the trough waveguide. A first and a second coaxial transmission line are coupled to the electromagnetic field near the junction of the trough waveguide and the rectangular waveguide.

This invention will be further described in conjunction with the accompanying drawings wherein:

FIG. 1 is a top view of a balanced mixer using a microwave hybrid in accordance with the present invention,

FIG. 2 is a cross-sectional view of the microwave hybrid mixer of FIG. 1 taken along lines 2-2 of FIG. 1,

FIG. 3 is a cross-sectional view of the microwave hybrid taken along lines 33 of FIG. 1, and

FIG. 4 is a cross-sectional view of the microwave hybrid of FIG. 1 taken along lines 44 of FIG. 1.

Referring to FIG. 1, there is illustrated a microwave hybrid made up of a first trough waveguide section 11 joined end to end at junction region 12 with a rectangular waveguide section 13. Extending perpendicular to the trough waveguide sec tion 11 near the junction region 12 with the rectangular waveguide section 13 are a pair of coaxial transmission line sections 15 and 17.

Referring to FIG. 2, there is illustrated in cross section the trough waveguide section 11 made up of conductive side walls 19 and 21 spaced by the conductive bottom wall 23. Referring to FIGS. 2 and 3, the height of side walls 19 and 21 changes a distance d from the junction of the trough waveguide 11 and the coaxial line sections 15 and 17. Extending halfway between the conductive side walls 19 and 21 from the bottom wall 23 is the center conductive fin 25. Above the center fin 25 and between the side walls 21 and 19 is located a conductive top wall 27. The width W of the bottom wall 23 is made about a quarter wavelength at the desired operating frequency of the trough waveguide to accommodate the narrow wall 37 of the rectangular waveguide. The bottom wall is made less than a half wavelength at the desired operating frequency to avoid second order modes. The center fin 25 height H is greater than a quarter wavelength at the desired operating frequency and is less than the height of the side walls 19 and 21. The height H, of the side walls 19 and 21 is less than a half wavelength at the desired operating frequency of the waveguide section 11.

Upon the application of RF electromagnetic waves at end 29 of the trough waveguide section 11, the electromagnetic waves propagate along the transmission line in a symmetrical or even mode. In the symmetrical or even mode, the electromagnetic field is primarily confined between the center fin 25 and the broad walls 21 and 19 with the electric field as indicated by arrows 31 and 33 extending in the opposite directions from the center fin 25. In the even mode, little or no appreciable electromagnetic field is exhibited a relatively short distance above the center fin 25 or in that portion of the trough waveguide near the conductive surface 27.

Referring to FIG. 4, there is illustrated rectangular waveguide section 13 made up of broad walls 35 and 36 spaced from each other by the narrow walls 37 and 39. This waveguide section is dimensioned to propagate electromagnetic waves therealong in the dominant asymmetrical TE mode. The height H of the broad walls 35 and 36 is greater than a half wavelength at the desired operating frequency of section 13. The width W of the narrow walls 37, 39 is about a quarter wavelength at the desired operating frequency. This width W is made less than a half wavelength at the desired operating frequency of the waveguide section 13. The electric field of the electromagnetic waves propagating at the desired frequency from end 39 toward the junction 12 with the trough waveguide 11 extends between the broad walls 36 and 35 as illustrated by arrow 41. The electric field extends all the way (arrow 41) across the waveguide from broad wall 35 to broad wall 36.

Referring to FIGS. 1 and 3, the trough waveguide section 11 is joined to the rectangular waveguide section 13 such that side walls 19 and 21 are joined end to end with the respective broad walls 36 and 35 of rectangular waveguide 13 to form a continuous conductive structure. The bottom wall 23 of trough waveguide section 11 is joined with the equal width narrow wall 37 of rectangular waveguide 13 to form a continuous structure. As mentioned previously, the height of the side walls 19 and 21 changes. From end 29 of trough waveguide 11 to a distance dfrom the coaxial lines 15 and 17, the height of the side walls 19 and 21 is H,. From the junction region 12 to the distance d from the junction of the coaxial lines 15 and 17 to the trough waveguide 11, the height of the side walls 19 and 21 is increased by height a so as to be equal to the height of the broad walls 35 and 36 of the rectangular waveguide 13.

The top wall 27 of trough waveguide 11 extends from end 29 toward the junction 12 of the two waveguide sections following the top edge of the side walls 19 and 21. The top wall 27 has a reflective step portion 38 therealong between the portion of the side walls 19 and 21 where the height of the walls is H, and where the height of the side walls 19 and 21 is H, a. The narrow wall 39 of the rectangular waveguide 13 joins top wall 27 at the junction 12 of the two waveguide sections 11 and 13. The step portion 38 of the top wall 27 acts as a partial reflective short at the upper part of the trough waveguide a distance d from the junction of the trough waveguide section 11 and the coaxial lines 15 and 17.

A first coaxial transmission line 15 extends from side wall 19 with the outer conductor 47 connected to the side wall 19. The inner conductor 48 of coaxial transmission line 15 extends through an aperture 49 in the side wall 19 into a trough 51 between the center fin 25 and the side wall 19. Inner probelike conductor 48 extends in the direction of the electric field in the trough 51.

In a similar manner the coaxial transmission line 17 is coupled to the trough 53 of symmetrical waveguide 11. The outer conductor 55 is directly connected to the side wall 21. The side wall 21 has an aperture 56 (dashed lines) therein and the inner conductor 57 extends through the aperture into the trough 53 of the trough waveguide 11. The trough is located between center fin 25 and sidewall 21. The probe-like inner conductor 57 extends in the direction of the electric field of the trough 53. The distance (d) the step portion 38 of the top wall 27 is located from the probe-like conductors 48 and 57 is about one-quarter wavelength at the operating frequency of the rectangular waveguide section 13 If it is desired to use the hybrid arrangement described above to provide a balanced mixer configuration, one end or port 60 of coaxial line is coupled to a diode mixer 61 and one end or port 62 of the coaxial transmission line 17 is coupled to a second diode mixer 65. The free end 29 of trough waveguide 11 is coupled to a source of RF (radio frequency) carrier signal, not shown. A local oscillator, not shown, is coupled across the free end 39 of rectangular waveguide 13.

in the operation of the balanced mixer arrangement shown in FIG. 1, signals such as RF signals received at port 29 propagate along the symmetrical trough waveguide 11 in the bound even mode with the electromagnetic field primarily between the center fin and the side walls 19 and 21. At the points where the probes 48 and 57 are located, the electromagnetic signals are equal in magnitude and in phase and therefore the signals coupled along symmetrical trough waveguide section 11 are equally divided and coupled along the transmission lines 15 and 17 to the respective mixer diodes 61 and 65. If the lengths of the transmission line sections 15 and 17 are equal, the RF signals coupled at port 29 arrive at the mixer diodes 61 and 65 in phase and are of equal magnitude.

If, as suggested above, a local oscillator is coupled at end 39, signals from the local oscillator at port 39 travel in the TE mode between the broad walls of the rectangular waveguide toward the junction 12 with the symmetrical trough waveguide. Since the coupling probe-like inner conductors 48 and 57 extend from opposite walls 19 and 21 which are coupled to respective opposite broad walls 36 and 35, the coupled signals are equal in magnitude but are 180 out of phase with respect to each other. Since the height of the top wall 27 of trough waveguide 11 beyond the reflective portion 38 is short compared to a half wavelength at the operating frequency of the local oscillator, the TE mode is cut off and RF port 29 is isolated from local oscillator signals applied at port 39. Increased coupling of the local oscillator signals coupled at port 39 and traveling along waveguide section 13 is provided by placement of the probe-like inner conductors 48 and 57 about a quarter wavelength at the local oscillator frequency from the reflective portion 38. it is known that when combining the mixer output signals, where the local oscillator signal applied to one of the mixer diodes is 180 out of phase with respect to the other mixer diodes, even order harmonic distortion from local oscillators is suppressible.

Isolation between the RF signal input port 29 and the local oscillator port 39 may be achieved in addition to the reduced height of top wall 27 or by the reflecting section 38. One such means is by placement of a lossy or resistant material 45 along the top wall 27 of the trough waveguide section or along the narrow walls 37 and 39 of the rectangular waveguide 13 as shown by dashed lines in FIGS. 2 and 3. RF impedance matching between the diode of the mixers 61 and 65 and a signal source may be achieved by a selective depth of penetration of the inner conductors 48 or 57 into the trough waveguide and by the position of the inner conductors 48 or 57 with respect to the reflecting section 38.

What is claimed is:

1. A microwave hybrid comprising:

a symmetrical trough waveguide section having a pair of conductive longitudinally extending side walls spaced by a longitudinally extending bottom wall and a shorter conductive center fin symmetrically disposed between said pair of side walls and extending from said bottom wall to form a first trough between the center fin and a first of said side walls and a second trough between the center fin and a second of the pair of side walls, said dimension and spacing of said side walls and said center fin being arranged to allow propagation of signals applied at one end of said trough waveguide section in the symmetrical even mode,

a rectangular waveguide section having conductive longitudinally extending broad walls and narrow walls, said dimensions and spacings of said broad walls and narrow walls being arranged to allow propagation of signals applied at one end of said rectangular waveguide section in the dominant asymmetrical TE mode,

said trough waveguide section and said rectangular waveguide section joined end-to-end with said first side wall forming a first continuous surface with one of said broad walls, said second side wall forming a second continuous surface with the other of said broad walls, and said bottom wall forming a third continuous surface with one of said narrow walls,

a first coaxial conductor coupled to that portion of the junction of said trough waveguide section and said rectangular waveguide section between the center fin and said first continuous surface,

a second coaxial conductor coupled to that portion of the junction of said trough waveguide section and said rectangular waveguide section between said center fin and said second continuous surface, and

means associated with said trough waveguide for providing isolation of said signals applied to said rectangular waveguide from the free end of said trough waveguide.

2. The combination claimed in claim 1 wherein said first and second continuous surfaces each have an aperture therein and said first coaxial conductor has the outer conductor connected to said first continuous surface and the inner conductor extending through said aperture in said first continuous surface and said second coaxial conductor has the outer conductor connected to said second continuous surface and the inner conductor extending through said aperture in said second continuous surface.

3. A microwave hybrid comprising:

a trough waveguide section having a pair of conductive longitudinally extending side walls spaced by a longitudinally extending bottom wall and a shorter conductive center fin symmetrically disposed between said pair of side walls and extending from said bottom wall to form a first trough between the center fin and a first of the pair of side walls and a second trough between the center fin and a second of the pair of side walls, said dimension and spacing of said side walls being arranged to allow propagation of said signals along said waveguide section in the symmetrical even mode,

a rectangular waveguide section having conductive longitudinally extending broad and narrow walls, said dimensions and spacings of said broad walls and narrow walls being arranged to allow propagation of signals applied at one end of said rectangular waveguide section in the dominant asymmetrical TE mode,

said trough waveguide section and said rectangular waveguide section joined end to end with said first side wall and a first broad wall fonning a first continuous conductive surface, said second side wall and a second broad wall forming a second continuous conductive surface, and said section of rectangular waveguide having a first longitudinally extending narrow wall joined end to end with said bottom wall to form a third continuous conductive surface with said bottom wall,

a first coaxial conductor having the outer conductor connected to the first of said pair of side walls and the inner conductor extending into the electric field region of said first trough near the junction region of said trough waveguide section and said rectangular waveguide section,

a second coaxial conductor having the outer conductor connected to the second of said pair of side walls and the inner conductor extending into the electric field region of said second trough near the junction region of said trough waveguide and said rectangular waveguide, and

means associated with said trough waveguide for providing isolation of said signals propagating along said rectangular waveguide from being coupled to the free end of said trough waveguide.

4. The combination claimed in claim 3 wherein said last mentioned means includes the height of said side walls over a portion thereof being substantially less than that of said broad walls and less then one-half wavelength at the frequency of said signals propagating along said rectangular waveguide.

5. The combination claimed in claim 4 wherein the height of said side walls changes from the shorter height to that equal to the length of said broad walls approximately one-quarter wavelength away from the coupling region of said coaxial conductors where said wavelength is at the frequency of said signals coupled to said rectangular waveguide.

6. The combination claimed in claim 5 wherein between said side walls and opposite said bottom wall is a conductive top wall which extends from the free end to the second narrow wall of said rectangular waveguide to form a fourth continuous conductive surface with said narrow wall and a reflecting short where the height of the side wall changes one-quarter wavelength away from the coupling point of the coaxial lines with the trough waveguide.

7. The combination as claimed in claim 3 wherein said last mentioned means includes lossy material located above the center fin between the side walls.

8. The combination claimed in claim 7 including lossy material adjacent to the narrow walls of said rectangular waveguide.

9. A balanced mixer responsive to RF microwave signals and a locally generated microwave signal comprising in com bination:

a symmetrical trough waveguide section having a pair of conductive longitudinally extending side walls spaced by a longitudinally extending bottom wall and a shorter conductive center fin symmetrically disposed between said pair of side walls and extending from said bottom wall to form a first trough between the center fin and a first of said side walls and a second trough between the center fin and a second of the pair of side walls, said dimension and spacing of said side walls and said center fin being arranged to support said RF microwave signals applied at one end of said trough waveguide section in the symmetrical even mode,

a rectangular waveguide section having conductive longitudinally extending broad walls and narrow walls, said dimensions and spacings of said broad walls and narrow walls being arranged to support said locally generated signals applied at one end of said rectangular waveguide section in the dominant asymmetrical TE mode,

said trough waveguide section and said rectangular waveguide section joined end to end with said first side wall forming a first continuous surface with one of said broad walls, said second side wall forming a second continuous surface with the other of said broad walls, and said bottom wall forming a third continuous surface with one of said narrow walls,

a first coaxial conductor coupled at one end to that portion of the junction of said trough waveguide section and said rectangular waveguide section between the center fin and said first continuous surface,

a second coaxial conductor coupled at one end to that portion of the junction of said trough waveguide section and said rectangular waveguide section between said center fin and said second continuous surface, and means including said side walls having a height less than one-half wavelength at said locally generated frequency for providing isolation of said locally generated signals applied to said rectangular waveguide from the free end of said trough waveguide,

a first mixer coupled to the free end of said first coaxial conductor, and

a second mixer coupled to the free end of said second coaxial conductor. 

1. A microwave hybrid comprising: a symmetrical trough waveguide section having a pair of conductive longitudinally extending side walls spaced by a longitudinally extending bottom wall and a shorter conductive center fin symmetrically disposed between said pair of side walls and extending from said bottom wall to form a first trough between the center fin and a first of said side walls and a second trough between the center fin and a second of the pair of side walls, said dimension and spacing of said side walls and said center fin being arranged to allow propagation of signals applied at one end of said trough waveguide section in the symmetrical even mode, a rectangular waveguide section having conductive longitudinally extending broad walls and narrow walls, said dimensions and spacings of said broad walls and narrow walls being arranged to allow propagation of signals applied at one end of said rectangular waveguide section in the dominant asymmetrical TE10 mode, said trough waveguide section and said rectangular waveguide section joined end-to-end with said first side wall forming a first continuous surface with one of said broad walls, said second side wall forming a second continuous surface with the other of said broad walls, and said bottom wall forming a third continuous surface with one of said narrow walls, a first coaxial conductor coupled to that portion of the junction of said trough waveguide section and said rectangular waveguide section between the center fin and said first continuous surface, a second coaxial conductor coupled to that portion of the junction of said trough waveguide section and said rectangular waveguide section between said center fin and said second continuous surface, and means associated with said trough waveguide for providing isolation of said signals applied to said rectangular waveguide from the free end of said trough waveguide.
 2. The combination claimed in claim 1 wherein said first and second continuous surfaces each have an aperture therein and said first coAxial conductor has the outer conductor connected to said first continuous surface and the inner conductor extending through said aperture in said first continuous surface and said second coaxial conductor has the outer conductor connected to said second continuous surface and the inner conductor extending through said aperture in said second continuous surface.
 3. A microwave hybrid comprising: a trough waveguide section having a pair of conductive longitudinally extending side walls spaced by a longitudinally extending bottom wall and a shorter conductive center fin symmetrically disposed between said pair of side walls and extending from said bottom wall to form a first trough between the center fin and a first of the pair of side walls and a second trough between the center fin and a second of the pair of side walls, said dimension and spacing of said side walls being arranged to allow propagation of said signals along said waveguide section in the symmetrical even mode, a rectangular waveguide section having conductive longitudinally extending broad and narrow walls, said dimensions and spacings of said broad walls and narrow walls being arranged to allow propagation of signals applied at one end of said rectangular waveguide section in the dominant asymmetrical TE10 mode, said trough waveguide section and said rectangular waveguide section joined end to end with said first side wall and a first broad wall forming a first continuous conductive surface, said second side wall and a second broad wall forming a second continuous conductive surface, and said section of rectangular waveguide having a first longitudinally extending narrow wall joined end to end with said bottom wall to form a third continuous conductive surface with said bottom wall, a first coaxial conductor having the outer conductor connected to the first of said pair of side walls and the inner conductor extending into the electric field region of said first trough near the junction region of said trough waveguide section and said rectangular waveguide section, a second coaxial conductor having the outer conductor connected to the second of said pair of side walls and the inner conductor extending into the electric field region of said second trough near the junction region of said trough waveguide and said rectangular waveguide, and means associated with said trough waveguide for providing isolation of said signals propagating along said rectangular waveguide from being coupled to the free end of said trough waveguide.
 4. The combination claimed in claim 3 wherein said last mentioned means includes the height of said side walls over a portion thereof being substantially less than that of said broad walls and less then one-half wavelength at the frequency of said signals propagating along said rectangular waveguide.
 5. The combination claimed in claim 4 wherein the height of said side walls changes from the shorter height to that equal to the length of said broad walls approximately one-quarter wavelength away from the coupling region of said coaxial conductors where said wavelength is at the frequency of said signals coupled to said rectangular waveguide.
 6. The combination claimed in claim 5 wherein between said side walls and opposite said bottom wall is a conductive top wall which extends from the free end to the second narrow wall of said rectangular waveguide to form a fourth continuous conductive surface with said narrow wall and a reflecting short where the height of the side wall changes one-quarter wavelength away from the coupling point of the coaxial lines with the trough waveguide.
 7. The combination as claimed in claim 3 wherein said last mentioned means includes lossy material located above the center fin between the side walls.
 8. The combination claimed in claim 7 including lossy material adjacent to the narrow walls of said rectangular waveguide.
 9. A balanced mixer responsive to RF microwave signals and a locally generated microwave signal comprising in combination: a symmetrical trough waveguide section having a pair of conductive longitudinally extending side walls spaced by a longitudinally extending bottom wall and a shorter conductive center fin symmetrically disposed between said pair of side walls and extending from said bottom wall to form a first trough between the center fin and a first of said side walls and a second trough between the center fin and a second of the pair of side walls, said dimension and spacing of said side walls and said center fin being arranged to support said RF microwave signals applied at one end of said trough waveguide section in the symmetrical even mode, a rectangular waveguide section having conductive longitudinally extending broad walls and narrow walls, said dimensions and spacings of said broad walls and narrow walls being arranged to support said locally generated signals applied at one end of said rectangular waveguide section in the dominant asymmetrical TE10 mode, said trough waveguide section and said rectangular waveguide section joined end to end with said first side wall forming a first continuous surface with one of said broad walls, said second side wall forming a second continuous surface with the other of said broad walls, and said bottom wall forming a third continuous surface with one of said narrow walls, a first coaxial conductor coupled at one end to that portion of the junction of said trough waveguide section and said rectangular waveguide section between the center fin and said first continuous surface, a second coaxial conductor coupled at one end to that portion of the junction of said trough waveguide section and said rectangular waveguide section between said center fin and said second continuous surface, and means including said side walls having a height less than one-half wavelength at said locally generated frequency for providing isolation of said locally generated signals applied to said rectangular waveguide from the free end of said trough waveguide, a first mixer coupled to the free end of said first coaxial conductor, and a second mixer coupled to the free end of said second coaxial conductor. 